1. Field of the Invention
The present invention relates to a method of irradiating a laser beam, and in particular, to a method of irradiating a laser beam that is used in forming an active layer of a thin film transistor or the like.
2. Description of the Related Art
Techniques of crystallizing a semiconductor film formed on an insulating substrate such as glass and techniques of increasing crystallinity thereof by using laser annealing have been researched widely in recent years. As a material for the semiconductor film, silicon (Si) is used in many cases. A technique of crystallizing a semiconductor film by using a laser beam, thus obtaining a crystalline semiconductor film, and a method of irradiating a laser beam to a semiconductor film, thus increasing crystallinity, are referred to as “laser crystallization” within this specification. Further, films that undergo laser irradiation during laser crystallization are referred to as “irradiation films”.
Compared to synthetic quartz glass substrates, which are often used conventionally, glass substrates have the advantages of low cost, good workability, and the ease with which large area substrates can be made. This is the reason that the aforementioned research is being carried out. Further, the reason that lasers are preferably used in crystallization is that the glass substrate melting point is low, and it is necessary to reduce the processing temperature to a temperature equal to or less than 600° C. Lasers can impart high energy only to a semiconductor film without causing the substrate temperature to increase very much. Further, a throughput is considerably high compared to a heating means that uses an electric heating furnace.
Crystalline semiconductor films formed by laser beam irradiation have high mobility, and therefore thin film transistors (TFTs) are formed using the crystalline semiconductor films. For example, the crystalline semiconductor films are utilized in an active matrix liquid crystal display device, or the like, in which TFTs used in a pixel portion, or TFTs used in the pixel portion and a driver circuit, are formed on one glass substrate.
A method of using a pulse oscillation laser (pulse laser) and a method of using a continuous wave laser (CW laser) exist as laser light sources used in laser crystallization. Excimer lasers such as XeCl lasers, and higher harmonics of solid lasers such as Nd:YAG lasers, Nd:YVO4 lasers, and Nd:YLF lasers may be used as laser light sources for the former method, and gas lasers such as Ar lasers, and higher harmonics of solid CW lasers such as Nd:YAG lasers and Nd:YVO4 lasers may be used as laser light sources for the latter method.
Amorphous silicon films (a-Si) and polysilicon films (poly-Si) are semiconductor films that can undergo laser crystallization, and the crystallinity of these non-single crystal semiconductor films is increased by performing laser crystallization.
However, if an a-Si film is formed by plasma CVD on a large area substrate, there is a problem in that the film thickness of the formed a-Si film varies according to location within the substrate surface due to the plasma distribution during film formation, the reaction gas outflow distribution, the temperature distribution of heated substrate, and the like, and a film thickness distribution thus develops. Further, if a poly-Si film is formed from a-Si, film thickness variations that occur during film formation of the a-Si film still remain.
For example, if an a-Si film is formed on a 600 mm×720 mm glass substrate by using plasma CVD, variations of ±5% of the a-Si film thickness develop within the substrate surface.
If a state exists in which the film thickness of the non-single crystal semiconductor film has dispersions depending on locations within the substrate surface, the energy necessary for crystallization in locations at which the film thickness has increased becomes relatively larger, and the energy necessary for crystallization in locations at which the film thickness has decreased becomes relatively smaller.
It is extremely difficult to control laser beam energy by the size of the film in thickness, and therefore only a fixed energy can be imparted to the irradiation film if laser crystallization is performed by using, for example, a pulse laser such as an excimer laser. The degree of crystallization thus differs depending on location within the substrate surface, and the grain size of the polycrystalline semiconductor film obtained becomes non-uniform in locations within the substrate surface. A problem therefore exists in that variations develop in the characteristics of TFTs formed on a large area substrate.
On the other hand, solid pulse lasers and solid CW lasers using solid laser media (hereinafter both are referred to together as solid lasers) are maintenance-free, have stable output, and are superior to excimer lasers in mass production because it is possible to have higher repetitive oscillation when using a solid laser as a pulse laser than when using an excimer laser.
A technique of laser crystallization for forming a polycrystalline silicon film having a large grain size of several tens of micrometers on a glass substrate by using a CW Nd:YVO4 laser with LD excitation has been developed recently. It is possible to manufacture TFTs having electron mobility equal to or greater than 600 cm2/Vs by using this technique. Forming an LSI containing a CPU on a glass substrate, to produce a “sheet computer”, is moving closer and closer to realization.
However, there are not many types of solid lasers at present, and almost all available solid lasers have an oscillation wavelength (fundamental wave) in the red color region or the infrared region. Semiconductor films absorb almost no light in the red color region or the infrared region, and therefore the second harmonic (2ω), the third harmonic (3ω), or a higher harmonic corresponding to a wavelength in the range of the visible light region to the ultraviolet light region is used when a solid laser is utilized during laser crystallization. However, the energy conversion efficiency with respect to the fundamental wave is highest with the second harmonic, and therefore it is advantageous from the perspective of an energy to use the second harmonic.
The wavelength of the second harmonic of a solid laser is mainly in the visible light region on the long wavelength side greater than 350 nm. The wavelengths of the second harmonic of typical solid lasers are shown as follows: Nd:YAG laser: 532 nm; Nd:YVO4 laser: 532 nm; Nd:YLF laser: 527 nm (or 524 nm); Ti:sapphire laser: 345 to 550 nm (variable wavelength); and Alexandrite laser: 350 to 410 nm (variable wavelength).
The skin depth to the semiconductor film is deep when using the second harmonic of a solid laser for laser crystallization compared to an excimer laser beam or the like having a wavelength in the ultraviolet light region, and therefore repetitive reflection develops within the semiconductor thin film, and there is interference between the reflected beam and the incident beam. The optical characteristics of the laser beam with respect to the irradiation film (reflectivity, transmissivity, and absorptivity) periodically fluctuate due to the film thickness of the semiconductor film due to the effect of the interference.
Refer to FIGS. 1A and 1B. FIG. 1A is a diagram showing absorption spectra of a-Si films formed on a glass substrate. Reference numeral 101 denotes an adsorption spectrum when the a-Si film thickness is 30 nm, reference numeral 102 denotes an adsorption spectrum when the a-Si film thickness is 50 nm, reference numeral 103 denotes an adsorption spectrum when the a-Si film thickness is 70 nm, reference numeral 104 denotes an adsorption spectrum when the a-Si film thickness is 90 nm, and reference numeral 105 denotes an adsorption spectrum when the a-Si film thickness is 110 nm. Further, FIG. 1B is a diagram showing absorption spectra of poly-Si films formed on a glass substrate. Reference numeral 111 denotes an adsorption spectrum when the poly-Si film thickness is 30 nm, reference numeral 112 denotes an adsorption spectrum when the poly-Si film thickness is 50 nm, reference numeral 113 denotes an adsorption spectrum when the poly-Si film thickness is 70 nm, reference numeral 114 denotes an adsorption spectrum when the poly-Si film thickness is 90 nm, and reference numeral 115 denotes an adsorption spectrum when the poly-Si film thickness is 110 nm.
It can be seen that the light absorption spectra are dependent upon the irradiation film thickness in the visible light region, on the long wavelength side greater than 350 nm. If laser crystallization is performed using a laser beam that possesses a wavelength in this wavelength region, then the energy imparted to the semiconductor film varies due to the film thickness of the semiconductor film itself, even if the laser beam energy is fixed.
Refer to FIGS. 2A and 2B. FIG. 2A is a diagram showing a film thickness dependence for light absorptivity by an a-Si film at a wavelength of 308 nm (reference numeral 201), and a film thickness dependence for light absorptivity by an a-Si film at a wavelength of 532 nm (reference numeral 202). FIG. 2B is a diagram showing a film thickness dependence for light absorptivity by a poly-Si film at a wavelength of 308 nm (reference numeral 211), and a film thickness dependence for light absorptivity by a poly-Si film at a wavelength of 532 nm (reference numeral 212).
As can be understood from FIGS. 2A and 2B, the skin depth to the semiconductor film is shallow in laser crystallization methods that use excimer lasers, such as an XeCl laser, because the laser beam wavelength is in the ultraviolet light region. Therefore there is no dependence of the light absorptivity on the irradiation film thickness, and there are no fluctuations of the energy imparted to the semiconductor film by laser irradiation due to the film thickness of the semiconductor film itself. On the other hand, if the film thickness of the a-Si film or the like fluctuates in laser crystallization methods that use the second harmonic of a solid laser, then the light absorptivity corresponding to the fluctuations is attenuated periodically and exponentially, and the energy imparted to the semiconductor film also fluctuates in a similar manner. It is therefore difficult to achieve uniform crystallization.
Laser crystallization methods that use the second harmonic of a conventional solid laser have been found to have more disadvantages in this point than laser crystallization methods that use an excimer laser beam.
The inventors of the present invention noticed that the dependence of the light absorptivity on the film thickness that can be seen for the second harmonic of a solid laser is rather effective against the problem of variations in crystallinity within the substrate surface caused by variations in the film thickness of the irradiation film within the substrate surface. In other words, by limiting the film thickness of the irradiation film, the energy absorbed becomes relatively larger at locations within the substrate surface when the film thickness of the irradiation film increases, and the energy absorbed becomes relatively smaller at locations within the substrate surface when the film thickness of the irradiation film decreases in laser crystallization using the second harmonic of a solid laser, and it is considered that crystallization can proceed to the same degree. It is thus expected that variations in the TFT characteristics within the substrate surface can be reduced.